Product Info

WipLL Radio Technology - Physical Layer System Description
2-2 Airspan Networks Inc. 25030311-08
2.1. Frequency Hopping Spread Spectrum
The WipLL system implements frequency-hopping code division multiple access
(FH-CDMA) spread spectrum modulation for digital signal transmission over the air
between the Base Station and the subscriber site. The WipLL system’s frequency
hopping supports a channel bandwidth of 1 MHz or 1.33 MHz, and channel spacing
of 1 MHz (or 1.75 MHz if operating in the 3.5 GHz band).
Frequency hopping is a basic modulation techniques used in spread spectrum signal
transmission. Spread spectrum enables a signal to be transmitted across a frequency
band that is much wider than the minimum bandwidth required by the information
signal. The transmitter "spreads" the energy, originally concentrated in narrowband,
across a number of frequency band channels on a wider electromagnetic spectrum.
In an FH-CDMA system, a transmitter "hops" between available frequencies
according to a specified algorithm, which can either be random or predefined (see
Figure 2-1). The transmitter operates in synchronization with a receiver, which
remains tuned to the same center frequency as the transmitter. A short burst of data
is transmitted on a narrowband signal. The transmitter then tunes to another
frequency, and transmits again. Therefore, the receiver is capable of hopping its
frequency over a given bandwidth several times a second (20 hops per second in the
WipLL system), transmitting on one frequency for a certain period of time, then
hopping to another frequency and transmitting again. The WipLL system supports a
hopping speed of 50 msec hopping intervals.
TIME TIME
1 1 2 2 3 3 4 4 5 5 6 6 7 7 88 99 1010 1111 1212
f1f1
f2f2
f3f3
f4f4
f5
f5
Frequency
Each channel
is 1 MHz wide
Figure 2-1: An example of Frequency Hopping Spread Spectrum